I know I have those transfer utils on HD somewhere but I can't lay my hands on them at the moment. Have you tried searching csdb for "transfer"?

Using a soundcard to sample it is not a good idea. The most reliable setup is using a DC2N, which unless you can find someone willing to sell to theirs is non-starter. Next best is probably MTAP with a suitable tape adaptor - but compared with a DC2N even that will be very unreliable.

If there are definitely undumped TAPs then I'm willing to dump them with my DC2N and return the tapes when done. However, a LOT of tapes have already been dumped - perhaps the TAPs already exists in the GB64 extras collection? If the TAPs exist however having them on PC won't help that much as there are no automated tools to make a single PRG/D64 from a TAP. You can get the raw program data from TAPCLEAN but you will need to reconstruct it - it's fairly easy with single loaders but a lot more complicated with multi-loaders.